1. Field of the Invention
The present invention relates to a photoelectric encoder, and in particular to a photoelectric encoder favorably used in incremental type encoders such as a linear encoder, a circular encoder and a rotary encoder and capable of improving the reproducibility of an origin position.
2. Description of the Related Art
An incremental type encoder in which a relative amount of movement of a scale with respect to a detector is detected by an increment has been widely used. The incremental type encoder is characterized in that it does not require arrangement of many tracks in a width direction orthogonal to a measuring direction of the scale, as compared with an absolute encoder which detects an absolute position, and is simple in constitution. On the other hand, immediately after the incremental type encoder is powered on or where a relationship thereof with respect to a reference position is deviated due to some reason, the deviation cannot be corrected by an incremental signal alone. Therefore, as shown in FIG. 1A (side view) and FIG. 1B (plan view), on a scale 10 in which a main track 14 for detecting an amount of movement is formed in a moving direction (measuring direction), there is installed an origin track 16 along with the main track 14. And, soon after the encoder is powered on, passage of the origin track 16 is detected to correct an incrementally measured position (Japanese Published Examined Patent Application No. H03-60041 (hereinafter, referred to as Patent Document 1), Japanese Published Examined Patent Application No. H03-60042 (hereinafter, referred to as Patent Document 2), Japanese Published Unexamined Patent Application No. H07-286861 (hereinafter, referred to as Patent Document 3), Japanese Published Unexamined Patent Application No. 2004-239829 (hereinafter, referred to as Patent Document 4), Japanese Published Unexamined Patent Application No. 2005-83808 (hereinafter, referred to as Patent Document 5). In the drawings, the reference numeral 20 denotes a detector which includes a light source 22, a light receiving element 24 for main signal arranged so as to oppose the main track 14 and a light receiving element 26 for origin signal arranged so as to oppose the origin track 16.
However, a conventional photoelectric incremental type encoder is independently provided on a scale 10 with a main track 14 for detecting a main signal and an origin track 16 for detecting an origin signal. Further, whereas the light receiving element 24 for main signal is constituted with a light receiving element array capable of reading a relative position, the light receiving element 26 for origin signal is to detect only whether the origin track 16 is present or not by referring to an amount of light received. Therefore, there is found such a problem that due to an optical variance of the tracks and an optical system as well as an electrical variance of a processing circuit, a phase deviation takes place between a main signal and an origin signal to easily cause lower reproducibility of an origin position.
Patent Document 1 has described the installation of an origin detecting portion within the main track. However, since the main track is discontinuous or interrupted by the origin detecting portion, it is impossible to prevent the phase deviation between the main signal and the origin signal.